Life in the Market Ecosystem, the second book in the Nature of Liberty trilogy, confronts evolutionary psychology head on. It describes the evolutionary psychologists' theory of gene-culture co-evolution, which states that although customs and culture are not predetermined by anyone's genetic makeup, one's practice of a custom can influence the likelihood of that person having children and grandchildren. Therefore, according to the theory, customs count as evolutionary adaptations. Extending that theory further, as entire systems of political economy-capitalism, socialism, and hunter-gatherer subsistence-consist of multiple customs and institutions, it follows that an entire political-economic system can likewise be classified as an evolutionary adaptation. Considering that liberal-republican capitalism has, insofar as the system has been implemented, done more to reduce the mortality rate and secure human fertility than other models of societal structure, it stands to reason that liberal-republican capitalism is itself a beneficent evolutionary adaptation. Moreover, as essential tenets of Rand's Objectivism-individualism, observation-based rationality, and peaceable self-interest-have been integral to the development of the capitalist ecosystem, important aspects of the Objectivism are worthwhile adaptations as well. This book shall uphold that position, as well as combat critiques by evolutionary psychologists and environmentalists who denounce capitalism as self-destructive. Instead, capitalism is the most sustainable and fairest political model. This book argues that of all the philosophies, Objectivism is the one that is most fit for humanity.